Inflatable well packers have been known for many years, the packers being used to isolate a zone in a well, so as, for example to enable a drill stem test to be performed, to perform a selective chemical treatment, or to isolate a redundant Zone in a productive well. There are presently two types of inflatable well packers, each being of a multilayered construction including an elastomeric inner bladder, but varying in the bearing system incorporated in the packer. The first type of known well packer includes wire or textile fibers, woven together with their ends secured to end fittings by an epoxy potting process, the sheath of woven wire or fibers being covered in an outer elastomeric boot which will form a hydraulic seal to the casing, or open-hole surface of a wall which the well packer will, in use, isolate. The other type of inflatable well packer utilises long, peripherally overlapping strips of spring steel which, when the packer is inflated, slide radially against each other like Venetian blinds, the strips surrounding the elastomeric inner bladder. The central portions of the strips are bonded to an outer annular elastomeric boot which acts as a hydraulic seal to the casing or open-hole surface in use of the well packer. An example of a well packer of this type is shown in U.S. Pat. No. 3160211.
With either of these known well packers there are a number of shortcomings. Firstly, the manufacture of either of these well packers is labour intensive. In the case of the woven sheath reinforced well packer, the reinforcing wire or fabric has to be hand-woven during assembly of the well packer. In the case of the spring steel strip reinforced well packer, the large number of overlapping strips are difficult to assemble and engage in their end fittings.
Furthermore, In either of the known types of well packers, the elastomeric inner bladder has to expand typically by twice the amount that the outer elastomeric boot has to expand. Thus if the packer has to be inflated by a ratio of 3:1 in order for the outer boot to make the required seal, the inner elastomeric bladder will have to expand by a ratio of 6:1. This results in the inner elastomeric bladder in its inflated state being very thin, thus making the inner elastomeric bladder susceptible to any micro faults which it may have in its structure. Furthermore, the high expansion ratio required by the inner elastomeric bladder severely limits the choice of materials which may be used. As a result of the limited choice of materials, the inner elastomeric bladder tends to have a very limited chemical resistance to any fluid other than water, thus limiting the life of the packer when the packer is exposed to fluids such as acids, solvents, diesel oil, and surfactants, these all being chemicals which are commonly required for treatments of zones within wells.
With regard to the outer elastomeric boot in either of the known types of well packers, although this typically only has to expand by a ratio of 3:1, and thus an increased choice of materials enables it to have a better chemical resistance to the fluids used in the wells than that of the inner elastomeric bladder, the outer elastomeric boot still has a relatively short life span.
Finally, there is a further drawback to such known inflatable well packers, in that when they are expanded they exert a high radial stress on the end fitting as they try to expand to a diameter greater than the diameter of the end fitting. This puts a complex compressive stress on the outer reinforcing members of either type of well packer, and furthermore, exerts a high tensile stress on the reinforcing members inner surface. In some cases this has resulted in a splitting of the end fitting, this then releasing the well packer into the well bore.